The present invention relates generally to porous bacteria filter media and is more particularly concerned with a new and improved sheet structure comprised of micron-diameter fibers and adapted for use in surgical face masks.
Fibrous sheet structures comprised predominantly of micron-diameter inorganic fibers suitably bonded to provide the necessary strength and structural integrity for use as filtering medium have been known for some time and have conventionally been prepared in accordance with well-known papermaking techniques. Unbonded inorganic filter papers, such as those made exclusively of glass fibers, generally exhibit low strength in a moisture laden atmosphere. This can be attributed at least in part to the straight, smooth-surfaced, rod-like nature of the glass fibers. In order to achieve a prolonged work life, it has been necessary to impart to the glass filter sheets sufficient strength and durability to retain their structural integrity during use. Accordingly, various bonding systems have been employed to achieve the required strength without disturbing the balance between porosity and filtering efficiency.
In the area of surgical face mask filtering media, it is essential that the filter exhibit a high degree of bacteria filtration efficiency, with the conventionally accepted and preferred minimum efficiency being about 96 percent. As will be appreciated, this high filtration efficiency must be coupled with breathability sufficient to permit use of the surgical face mask without undue discomfort to the user. Although the mechanism of bacteria filtration efficiency has not been fully explored, some workers contend that it is associated with the network of fine glass fibers used in the filter and that the interception and retention of bacteria is controlled in large part by the presence of fine diameter fibers, which can form a closely spaced network of fine pores. The presence of binder in the sheet tends to cover over a fraction of the finer pores in this fiber network, and this in turn shifts the pore size distribution of the filter toward the larger size range, thus reducing the filter's efficiency. Others believe it is associated with an electrostatic charge on the micron-diameter glass fibers that attracts and affixes the particles within the sheet structure. The total charge on the filtering medium is believed to be controlled to a limited degree by adjusting the fiber dimension and thereby adjusting the total exposed surface area carrying an electrostatic charge. The charge is controlled predominantly by the amount of fine glass fibers. Under either theory a reduction in the amount of binder used in a filter sheet can result in an improvement in the filtration efficiency at comparable weight levels. Alternatively, a reduction in the basis weight of the filter material could be made while maintaining the requisite bacteria filtration efficiency.
Typical examples of porous inorganic fiber sheet material can be found in U.S. Pat. Nos. 3,253,978 and 3,594,993 although these patents do not refer specifically to surgical face mask filtering media having a high bacteria filtering efficiency. The foregoing patents describe bonding systems wherein nonfibrous bonding agents are incorporated into the web material either during web formation or as a postformation treatment.
Fibrous binder systems using heat sealable fibers, such as vinyl copolymer fibers, e.g. vinyon fibers, have been used for filtering media having surgical face mask application. These all-glass fiber sheets have required the use of a minimum of about 30 percent by weight of vinyon to achieve appropriate sheet machinability and strength. Additionally, difficulty has been experienced with such material due to the low softening point, i.e. about 140.degree.-160.degree. F, of the vinyon fibers resulting in the material adherring to the surface of the hot dryer cans used in the conventional papermaking process. This has tended to cause binder build-up in the dryer section and resulted in frequent shutdown periods during manufacturing runs in order to remove the undesirable build-up from the equipment. Attempts to reduce the amount of vinyon fibers below the 30 percent minimum level have resulted in a substantial loss in strength as well as a reduction in the porosity of the resultant sheet material.
Recently, films and web material have been produced from polyolefin fibers of high molecular weight and low melt index, that is an average molecular weight greater than 200,000 and a melt index of less than 1.0 and preferably less than about 0.5. The fibers, referred to as synthetic wood pulp, are to be contrasted with conventional polymeric staple fibers made by melt spinning operations. These high molecular weight polyolefin fibers cannot be processed to fibers by the conventional spinning technique due to the lack of ready flowability of this material. Instead they are formed by techniques such as shear precipitation. Webs produced from these fibers have been composed predominantly of the polymeric material and usually are fused under appropriate temperature conditions to form thin films. A typical example of such a process can be found in British Pat. No. 1,386,982. However, it has been reported that when these fibers are mixed with papermaking fibers, the tensile strength and apparent density of the resultant product decreases as the polymer fiber content increases.
Accordingly, it is an object of the present invention to provide a new and improved filter medium well suited for use in surgical face masks that permits a substantial reduction in the basis weight of the material without substantially reducing the bacteria filtration efficiency or the breathability of the material. Since the filter medium accounts for about 60 percent of the face mask's total weight, a lighter medium significantly contributes to improved comfort particularly during long surgical procedures.
Another object of the present invention is to provide a new and improved filter medium for surgical face masks that exhibits the aforementioned bacteria filtration efficiency and breathability while reducing the thermoplastic content and basis weight of the material and improving the stiffness of the mask thereby maintaining the mask in a semi-rigid state out of contact with the nose and lips of the wearer to provide increased comfort during use.
Still another object of the present invention is to provide a nonwoven inorganic fiber filter media that permits adjustment in the strength without adversely influencing the porosity and filtration efficiency and facilitates either an increase in the total surface area of the glass fibers within the media or a reduction in the weight thereof while maintaining a high level of bacteria filtration efficiency. Included in this object is the provision for a reduction in the basis weight of the material with its accompanying benefits in both performance and economy of manufacture without sacrificing strength and while achieving additional web rigidity.
A further object of the present invention is to provide a new and improved filter medium of the type described that provides greater processability while obviating binder build-up on the hot dryer cans of the papermaking apparatus and greater control over the fusion characteristics of the heat sealable binder used in the filter medium. Included in this object is the provision for using high molecular weight polyolefin fibers while surprisingly obtaining improved tensile strength and only a minor variation in permeability properties as the amount of polyolefin in the material increases.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
These and related objects are accomplished in accordance with the present invention by providing a porous bacteria filter medium in flexible fibrous sheet form comprised of inorganic micron-diameter fibers and containing less than about 30 percent by weight of a polyolefin heat sealable binder fiber randomly dispersed throughout the sheet and fused therein. This filter medium has a bacteria filtration efficiency of at least 96 percent and sufficient porosity to permit breathability within comfortable limits over prolonged periods of time. The polyolefin fibers used in the sheet exhibit a surface area of at least 1 square meter per gram and are formed of high molecular weight, low melt index polymeric material.
A better understanding of the invention will be obtained from the following detailed description wherein the article of manufacture possessing the features, properties and relation of elements described and exemplified herein.